1. Field of the Invention
The present invention concerns a suck back valve which is capable of preventing liquid drip of a liquid at a supply port, for example, by sucking a predetermined amount of the liquid flowing through a fluid passage under a displacement action of a diaphragm.
2. Description of the Related Art
A suck back valve has hitherto been used, for example, in a semiconductor wafer manufacturing process. The suck back valve, when halting the supply of a coating liquid onto the semiconductor wafer, possesses a function for preventing so-called "liquid drips" wherein minute amounts of coating liquid continue to drip from the supply port toward the semiconductor wafer.
FIG. 3 illustrates a suck back valve in accordance with this conventional technique which may be referred to, for example, in Japanese Utility Model Publication No. 8-10399.
The suck back valve 1 includes a valve body 5 which is formed by a fluid passage 4 intercommunicating a fluid inlet port 2 and a fluid outlet port 3, and a bonnet 6 which is attached to an upper part of the valve body 5. A diaphragm 7 which is constructed from a thick-walled portion and a thin-walled portion, is disposed centrally within the fluid passage 4. A pressurized fluid supply port 8, which is connected to a pressurized fluid supply source 13, and which supplies pressurized air for operating the diaphragm 7 under a switch-over action of an unillustrated directional control valve, is formed in the bonnet 6.
A piston 9 is fitted to the diaphragm 7, the piston 9 having a v-packing 10 attached thereto which is slidable along an inner wall surface of the valve body 5, together with performing a sealing function. Further, a spring 11 which normally biases the piston 9 in an upward direction, is disposed in the valve body 5.
Further, reference numeral 12 shows a screw member which abuts against the piston 9 and functions as a stop for regulating a displacement amount of the piston 9, thereby adjusting a flow amount of coating liquid which is sucked by the diaphragm 7 by increasing or decreasing a screw-in amount thereof.
A coating liquid supply source 14 is connected to the fluid inlet port 2 through a conduit 15 such as a tube or the like. Further, an ON/OFF valve 16, which is constructed separately from the suck back valve 1, is connected between the coating liquid supply source 14 and the fluid inlet port 2. By energization and de-energization thereof, the ON/OFF valve 16 performs a function of switching between a supply state and a supply-halted state of the coating liquid with respect to the suck back valve 1.
Next explaining in outline the operation of the suck back valve 1, in an ordinary state in which a coating liquid is supplied from the fluid Inlet port 2 to the fluid outlet port 3, the piston 9 and diaphragm 7 are displaced downward in unison under an action of pressurized air which is supplied from the pressurized fluid supply port 8. The diaphragm 7 which is connected to the piston 9 projects into the fluid passage 4, as shown by the two-dot-chain line in FIG. 3.
At that point, in the event that the flow of coating liquid inside the fluid passage 4 is halted under a switching action of the ON/OFF valve 16, by terminating the supply of pressurized air from the pressurized air supply port 8, the piston 9 and diaphragm 7 are raised upwardly in unison under action of an elastic force of the spring 11, wherein any coating liquid remaining inside the fluid passage 4 is sucked back under a negative pressure action of the diaphragm 7, thereby preventing unwanted liquid drip from occurring at an unillustrated supply port.
Notwithstanding, with the suck back valve 1 according to the conventional technique, an upper-limit position of the piston 9 which is raised under an action of the elastic force of the spring 11 is regulated by abutment with an end of the screw member which functions as a stop. However, in this case, vibrations are generated as a result of kinetic energy of the piston. Thus, a disadvantage occurs whereby such vibrations are conveyed to the fluid passage 4 and effect the coating liquid which remains in the supply port (not shown), causing unwanted dripping of coating liquid onto the semiconductor wafer.
Further, the suck back valve according to the conventional technique is constructed so that, by raising the piston 9 by action of the elastic force of the spring 11, a fixed amount of coating liquid remaining in the fluid passage is sucked back under a negative pressure action of the diaphragm. Accordingly, the rate of speed at which the piston 9 is raised is set and determined by the elastic force of the spring 11. Thus, for example, a disadvantage results in that the rate of speed at which the piston is raised is not freely controllable corresponding to characteristics of the fluid flowing in the fluid passage 4.
Furthermore, with the suck back valve 1 according to the conventional technique, troublesome pipe-connecting operations are needed between the suck back valve 1 and the ON/OFF valve 16, along with the need for a dedicated space for installation of the ON/OFF valve 16 externally of the suck back valve 1, with the disadvantage that installation space increases.
Still further, fluid passage resistance increases due to the piping which is connected between the suck back valve 1 and the ON/OFF valve 16, resulting in deterioration of the response accuracy (responsiveness) of the diaphragm 7.